Operation of engines such as internal combustion engines produces a variety of exhaust gas emissions, including carbon dioxide, carbon monoxide, unburned hydrocarbons and various nitrogen-oxygen compounds, known in the art collectively as “NOx.” In recent years, certain jurisdictions have implemented plans to reduce allowable limits on the relative amounts of certain exhaust gas emissions of particular concern. The present and upcoming restrictions have prompted engineers to further research known engine and exhaust systems, and have driven a search for new strategies for exhaust gas treatment and engine operation showing promise for a reduction in certain emissions.
Control over the relative amounts of various compounds in engine exhaust gas has been attempted in numerous different ways. In some four-cycle engines, for example, the relative timing of intake, exhaust, compression and expansion strokes are varied to control combustion phasing within the engine cylinders and therefore affect the relative amounts of certain emissions. In other designs, combustion exhaust is treated by chemical means in an aftertreatment system to generate a desired emissions profile.
One technology that has received increasing attention involves recirculation of exhaust gas back into the engine intake pathway, known in the art as “EGR.” In such an approach, a portion of the relatively inert exhaust gas from the engine's exhaust manifold is returned to the intake manifold. Among other things, the addition of exhaust gas to the fuel and air mixture in an engine cylinder can provide an inert gas heat sink that allows combustion to proceed at a relatively cooler temperature than might otherwise be practicable. EGR-related technologies have become increasingly prevalent in diesel engines in particular.
The exhaust gas recirculation apparatus, commonly known as an EGR loop, may include a valve-controlled passage extending between an exhaust system of the engine and the intake manifold. In certain instances, it has been found to be desirable to cool the recirculated exhaust gas prior to supplying it to the intake manifold. In turbocharged engines, it may be desirable to combine exhaust gas with combustion air upstream from the compressor. When the exhaust gas and air mixture is compressed via the turbocharger compressor, it will increase in temperature. Cooling the exhaust gas prior to compression allows the temperature of the mixture ultimately delivered to the intake manifold to be maintained at more manageable and more desirable levels than would occur where the exhaust gas arrives at the compressor without first being cooled.
Various exhaust gas cooling devices are known which are disposed within the EGR loop. One common device includes an apparatus known in the art as a “tube-in-shell” heat exchanger. A conventional tube-in-shell heat exchanger includes a passage for exhaust gas, which is thermally coupled with one or more passages carrying cooling water or engine coolant. The engine coolant/water is circulated through the heat exchanger, exchanging heat with the exhaust gas passing therethrough, and thenceforth returned to the engine radiator for cooling in a conventional manner.
While designs similar to those above have experienced a certain level of technical and commercial success, there are limitations to the efficacy and durability of conventional heat exchangers. For instance, a coolant or water-cooled heat exchanger requires a relatively complex plumbing system connecting with the main radiator assembly of the engine. In addition, a typical engine radiator can be limited in its capacity to dissipate heat from the engine. Adding a subsystem such as an exhaust gas cooler can increase the heat load of the radiator above that which is desirable. Heightened emissions regulations have further compounded this problem, as in certain operating strategies it is actually desirable to reject more heat from the engine than was traditionally required. While some manufacturers have attempted to increase the size of engine radiators to provide more heat rejection capacity, engines and associated cooling systems are commonly sized and positioned in such a way that they cannot readily be reconfigured to accommodate larger radiators. Moreover, increasing the size of radiators, as well as using overly large and heavy conventional exhaust gas coolers adds undesirable weight to a work machine using such an engine as its power source.
Yet another problem plaguing engineers is the tendency for conventional exhaust gas coolers to suffer degradation due to corrosive, condensed exhaust gases. As exhaust gases are cooled, a certain proportion of the gases may actually condense to an acidic liquid. Such liquids are known to be corrosive to metals, and can reduce the effectiveness of a conventional exhaust gas cooler as well as reduce its operating life.
Another problem with engine coolant based EGR coolers are limitations on exhaust gas exit temperatures which may be imposed by the engine coolant. The engine coolant will often be maintained at a minimum temperature by an engine coolant thermostat, often in the range of 85° C. to 90° C. This effectively limits the temperature of exhaust gas exiting the exhaust gas cooler to a range above about 90° C. An air based EGR cooler can provide lower temperatures for exhaust gases leaving the cooler and entering the turbocharger, which can result in greater air density, and hence better engine performance. Typical air cooled exhaust gas heat exchangers, like an air to air aftercooler, depend upon a cooling flow of 5 lb or more per 1 lb of hot gas to provide the required cooling function.
Still another problem with engine coolant based EGR coolers is the potential for engine coolant to leak into the exhaust gas passages, or for exhaust gases to leak into the engine coolant passages. In either case, leakage of one fluid into another passage can be detrimental to the life of the engine.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.